Water-absorbing resin (super absorbent polymer [SAP]) is a water-swellable, water-insoluble polymer gelling agent. Water-absorbing resin is heavily used mainly for disposable purposes, i.e., for absorbent articles such as a disposable diaper and a sanitary napkin and for an agricultural and horticultural water retaining agent, an industrial waterproofing agent, and the like. Of such water-absorbing resins, a polyacrylic acid (salt)-based water-absorbing resin, in which acrylic acid and a salt thereof are used as monomers or a combination of a polyacrylic acid-based water-absorbing resin and a polyacrylic acid salt-based water-absorbing resin, is particularly industrially most widely used from the viewpoint of its high water absorption performance.
Along with the advancement of the performance of a disposable diaper, which is a main application of use of the water-absorbing resin, the water-absorbing resin is required to have various functions (physical properties). Specific examples of the physical properties of the water-absorbing resin include not only merely a high fluid retention capacity but also a gel strength, a water-soluble component, a water absorption speed, a fluid retention capacity under pressure, a liquid permeability, a particle size distribution, a urine resistance, an antibacterial property, an impact resistance (damage resistance), a powder fluidity, a deodorizing property, an anti-coloring property (whiteness), a low dustiness, and the like.
Of the physical properties mentioned above, the liquid permeability is considered to be a more important physical property in view of an increase in the amount (for example, 50 weight % or more) of a water-absorbing resin used in a disposable diaper. Further, in addition to the liquid permeability, the water absorption speed is considered to be an important fundamental physical property for a water-absorbing resin. Under the circumstances, there have been studied techniques for improving the liquid permeability of, preferably both the liquid permeability and the water absorption speed of, a water-absorbing resin.
Patent Literature 1 studies a method for producing polyacrylic acid (salt)-based water-absorbing resin powder having both a higher liquid permeability and a higher water absorption speed. Specifically, Patent Literature 1 studies control of a gel-crushing step, a drying step, and a surface treatment step that are included in a process for producing polyacrylic acid (salt)-based water-absorbing resin powder.
Patent Literature 2 discloses a method including an air-bubble forming and containing step of decreasing the solubility of dissolved gas in an aqueous monomer solution in the presence of a surfactant and/or a dispersing agent to form air bubbles in the acrylic acid-based aqueous monomer solution so that the aqueous monomer solution contains the air bubbles. The method intends to provide white water-absorbing resin powder that is high in both liquid permeability (for example, SFC) and water absorption speed (for example, FSR).
Patent Literature 3 discloses a method including a step of polymerizing, in the absence of a surfactant or in the presence of not more than 300 ppm of a surfactant, an acrylic acid-based aqueous monomer solution containing gas dissolved and/or dispersed therein by a predetermined method. The method of Patent Literature 3 intends to highly efficiently produce a water-absorbing resin with a high water absorption speed without impairing the liquid absorbing property of a sanitary article or the like to be produced and without excessively decreasing the bulk specific gravity.
Patent Literature 4 discloses a method for producing water-absorbing resin particles that are excellent in absorption capacity and centrifugal absorption capacity under load through a step of mixing a chlorine- or bromine-containing oxidizing agent with a polymerization mixture for producing a crosslinked hydrogel or with a crosslinked hydrogel and a step of heat-treating (heating) the resulting mixture.
Patent Literature 5 discloses a method including copolymerizing a compound prepared by neutralizing acrylic acid or a methacrylic acid and a crosslinking agent having two or more polymerizable unsaturated groups, the copolymerizing including first adding a radical polymerization initiator in an amount of 0.0001 mol % to 0.1 mol % per the total amount of monomers, and then adding the radical polymerization initiator in an amount of 0.01 mol % to 5 mol % in total per the total amount of monomers in one or more portions for polymerization. The method intends to easily and inexpensively produce a hydrogel that is capable of absorbing a large amount of water rapidly and retaining the water and contains only a small amount of unreacted monomer.